The invention relates generally to flame-retardants for polymeric compositions. More particularly, it relates to liquid flame-retardants for urethane polymers (polyurethanes).
Polyurethanes are organic polymers containing repeated urethane linkages (R.sub.1 NHCOOR.sub.2). The most common commercial method of forming these polymers is by the reaction of polyfunctional hydroxy compounds with polyfunctional isocyanates. The structure of a polyurethane derived from a dihydroxy compound OH--R--OH and a diisocyanate is given by ##STR2##
Cellular polyurethanes with properties ranging from rigid to flexible foam products are normally prepared from diisocyanates and hydroxyl-terminated polyether polyols or polyester polyols. Linear or only slightly branched polyols are used to provide flexible foams, whereas more highly branched polyols produce rigid foams. Foaming may be accomplished by including water in the system, the reaction between isocyanate and water providing carbon dioxide for foaming. Alternatively, foaming may be accomplished by including in the system a low-boiling liquid such as trichlorofluoromethane as a blowing agent. Appropriate catalysts and stabilizers control the foam formation and cure. A more detailed description of chemistry and technology of urethane polymers can be found in Kirk-Othmer, Encyclopedia of Chemical Technology, 2d edition, volume 21, pp. 56-106, Interscience Publishers, (1970); Encyclopedia of Polymer Science and Technology, volume 5, pp. 445-479, Interscience Publishers, (1971); both of which are herein incorporated by reference.
Polyurethane foams have excellent strength, durability, low density, light color, uniform cell size, and good heat insulating properties. Unfortunately, the foams have little inherent resistance to burning. In view of the extensive commercial use of polyurethane foams in the fields of insulation, structural reinforcement, cushioning, upholstery, electrical encapsulation, and the like, there has been an extensive effort in the polyurethane industry to produce a flame-retardant foamed product. Consequently, many organic and inorganic compounds have been employed as flame-retardants. However, none of these compounds has proved to be entirely satisfactory for this purpose.
One class of flame-retardants which has been incorporated into rigid and flexible polyurethane foams has been nonreactive organo-phosphorus compounds. Examples of such compounds include the cyclic phosphonate esters described in U.S. Pat. Nos. 3,789,091 and 3,849,368, the halogenated phosphonate-phosphite compounds described in U.S. Pat. No. 3,014,956; and the well-known tris-(2,3-dibromopropyl)phosphate. Such organo-phosphorus compounds have occasionally had a deleterious effect upon the physical properties of the foamed polyurethane product, e.g. moderate to severe foam discoloration. Moreover, some flame-retardant organo-phosphorus compounds possess undesirable toxicity problems. Other compounds, such as the cyclic phosphonate esters, have very high viscosity which render them difficult to use in standard foam-dispensing equipment due to pumping and mixing problems.
Another class of flame-retardants used in polyurethane foams has been inorganic and organic halogen-containing compounds. These compounds have included the bromohydrins of pentaerythritol such as, for example, 2,2-bis(bromomethyl)-1,3-propanediol, commonly known as dibromoneopentyl glycol ("DBNPG"), 3-bromo-2,2-bis(bromomethyl) propanol, commonly known as tribromoneopentyl alcohol ("TBNPA"), 2-bromomethyl, 2-hydroxymethyl, 1,3,-propanediol, and mixtures thereof. The use of both DBNPG and TBNPA as flame-retardant additives in rigid and flexible polyurethane foams is described in U.S. Pat. Nos. 3,738,953; 3,773,696; 3,933,693; and 4,052,346.
The use of the bromohydrins of pentaerythritol as flame-retardants for polyurethane foams has many inherent advantages over the use of other flame-retardants. First, the bromohydrins of pentaerythritol have excellent resistance to degradation by the urethane catalyst. Second, the bromohydrins of pentaerythritol enter into the polymerization reaction and become bound as part of the polymer matrix. Thus, they are not readily removed from the foam by volatilization, leaching, or migration. Third, the bromohydrins of pentaerythritol supply flame-retardancy to polyurethane foams without adding significantly to the smoke level. Fourth, the bromohydrins of pentaerythritol are compatible with the various other components generally employed in the production of polyurethane foams. Finally, the polyurethane foams containing the bromohyrins of pentaerythritol are generally resistant to discoloration or scorching and have excellent physical and mechanical properties.
However, one of the major drawbacks to the use of the bromohydrins of pentaerythritol is that both DBNPG and TBNPA are relatively high melting solids (melting at 109.degree.-110.degree. C. and 68.degree.-69.degree. C., respectively). Consequently, DBNPG and TBNPA may undergo some decomposition at temperatures above their melting point if not held under the proper conditions. Because of this, they are usually handled in a powdered or flake form. This severely limits the processing of these compounds in standard foam-dispensing equipment because of the difficulty of keeping the compounds homogeneously suspended in the foam component without excessively increasing the viscosity of the resinous ingredients. Very high viscosity renders most commercial foam formulations impossible to use in practice due to pumping and mixing problems.
U.S. Pat. Nos. 3,933,693 and 4,052,346 suggested a method of alleviating the solids problem by dissolving or dispersing DBNPG or TBNPA in the polyfunctional hydroxy compound used in the urethane polymerization reaction. However, this procedure is undesirable because the solubility of DBNPG and TBNPA in most polyfunctional hydroxy compounds is so extremely limited that excessive amounts of the polyfunctional hydroxy compounds need be employed in order to introduce flame-retardant amounts of the DBNPG or TBNPA into the polyurethane foam. (For example, U.S. Pat. No. 3,933,693 teaches that DBNPG has a maximum solubility of only about 15 percent by weight in a polyether polyols such as Multranol.RTM. 7100 or 3900). Moreover, the polyfunctional hydroxy compounds most suited as liquid carriers and/or solvents for DBNPG and TBNPA oftentimes are not the most desirable polyfunctional hydroxy compounds for the urethane polymerization reaction.
U.S. Pat. Nos. 3,789,091 and 4,052,346 suggested blending conventional flame-retardants such as the cyclic phosphonate esters and a flame-retardant having free --CH.sub.2 OH or CH.sub.2 CH.sub.2 OH groups to produce a synergistic flame-retardant for polyurethane foams. However, it has not heretofore been generally desirable in the polyurethane foam industry to employ such a blend for although the flame-retardancy of the blend may be synergistically enhanced, the physical form of each of the materials, e.g. a solid or extremely viscous liquid, poses difficult material handling problems. Consequently, the blended material would seemingly not offer any of the formulating advantages of normal low viscosity liquid flame-retardants.
It would therefore be desirable to provide a flame-retardant which has not only a high concentration of the desirable bromohydrins of pentaerythritols, but which also offers the formulating advantages of being a low viscosity liquid at ambient temperature and of being compatible with the various other components generally employed in the production of polyurethane foams.